A conservative outside funding group backed by Charles and David Koch is pulling back from the Wisconsin Senate race, canceling more than $2 million in TV ads it had originally planned to run in support of Republican Sen. Ron Johnson .

The decision by the Freedom Partners Action Fund is an ominous sign for Johnson, who is locked in a tight race against former Democratic Sen. Russ Feingold. The first-term incumbent is widely considered one of the two most vulnerable Republican senators this year (along with Illinois Sen. Mark S. Kirk), and early polls of the race have shown him trailing Feingold.

"We are realigning our television advertising strategy to ensure maximum impact across key Senate races," said James Davis, a spokesman for Freedom Partners.

Davis added that Freedom Partners will maintain an on-the-ground voter outreach effort in Wisconsin, as well as fund digital media and direct mail.

But the decision to cancel what was, according to media tracking sources, a planned $2.2 million reservation, spread over nine weeks, will fuel concern about Johnson's viability, especially as it comes a day after the National Republican Senatorial Committee delayed its own ad buy in the state until October.

A spokesman for the Johnson campaign pushed back on those concerns, saying that the campaign is in as good a shape as it's ever been.

"We just had our strongest fundraising quarter ever and the polls show this race tight," said Brian Reisinger, a Johnson campaign spokesman. "We are comfortable and confident and believe we have the support to run a winning campaign. The voters already fired Sen. Feingold once, and they will reject him again."

Johnson recently had his strongest fundraising report to date, raising $2.8 million in the year's second quarter. And polls have shown him gaining ground on Feingold after initially trailing by a double-digit deficit.